There now exists overwhelming evidence that a high percentage of so-called undifferentiated diarrheas in man are caused by enterotoxin-producing Escherichia coli. These E. coli generally do not exhibit enteropathogenic (EEC or EPEC) serotypes. At the present time identification of enterotoxigenic E. coli from human sources is solely dependent on assays for enterotoxin production and even this is limited to investigatory situations. Enterotoxigenic E. coli from animal sources can also be associated with particular surface-associated antigens such as K88 which impart the ability to colonize the small bowel and hence are true virulence factors. We have recently demonstrated the existence of a surface-associated colonization factor on E. coli enterotoxigenic for man. This colonization factor is a heat-labile antigen visible in electron micrographs as a pilus-like structure. The colonization factor resembles the K88 antigen but is immunologically distinct from K88. Colonization factor activity is readily demonstrable in the infant rabbit model. Employing this model we have shown that the ability of toxigenic E. coli to colonize the infant rabbit intestine is neutralizable with specific antiserum. We have also found the colonization factor to be plasmid-associated. We propose to further characterize the colonization-associated antigen in the cell-associated and in the purified state. Also, several assay methods are available for screening fresh human toxigenic E. coli isolates so that the possible relationship between this or similar virulence factors and human E. coli diarrhea can be ascertained. Concurrently, we propose to investigate the immunological aspects of E. coli virulence factors (i.e. enterotoxin and colonization factor) employing suitable animal models. The ultimate goal of this project is the development of methods for the rapid identification of virulent enterotoxigenic E. coli and prevention of enterotoxigenic E. coli diarrhea in man via immunological protection.